Astron. Astrophys. 329, L45-L48 (1998)

2. Observations and data reduction

The NGC 1068 speckle interferograms were obtained with the 6 m telescope at the Special Astrophysical Observatory (SAO) in Russia on October 3, 1996. The speckle interferograms were recorded with a 256 256 pixel NICMOS 3 camera through a standard K-band filter with center wavelength of 2191 nm and FWHM bandwidth of 411 nm. A typical speckle frame is shown in Fig. 1. The exposure time per frame was 200 ms, K-band seeing was about 1:005. 778 object speckle interferograms and 526 reference star (HIC 12014) interferograms were recorded. The scale was 30.82 mas/pixel and the field of view 7:009 7:009. Diffraction-limited images were reconstructed from the speckle data using the speckle masking method (Weigelt 1977; Lohmann, Weigelt, Wirnitzer 1983; Pehlemann, Hofmann, Weigelt 1992). The process includes the calculation of the average power spectrum and the average bispectrum and the subtraction of the detector noise terms from those. The 4-dimensional bispectrum of each frame consisted of 49 million elements. No postprocessing by image restoration methods was applied to the speckle masking reconstructions. For the calibration of the flux the photometric standard star HIC 110609 ( BS 8541), chosen from Elias et al. (1982), was observed on September 30, 1996. We adopt the HIC 110609 flux to be 12.57 Jy at 2.2µm.

Fig. 1. One of our 778 speckle interferograms of NGC 1068 taken through a K-band filter, demonstrating that it is possible to obtain speckles with high signal-to-noise ratio from a K source, even under not very good seeing conditions. The observations were made using a NICMOS 3 array with 200 ms exposure time per frame and multiple read-out (4 ). Furthermore, the high-contrast speckles show that the NGC 1068 core is very compact since the speckles look like speckles of a point source. The shown field of view is 1:0085 1:0085.

© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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